A terahertz wave is an electromagnetic wave having a frequency of approximately 0.01 THz to 100 THz corresponding to an intermediate range between light waves and radio waves, and has an intermediate property between light waves and radio waves. As an application of such a terahertz wave, a technology for acquiring information on a subject by measuring a temporal waveform of an electric field amplitude of a terahertz wave which is transmitted through or is reflected by the subject has been studied (refer to Patent Document 1).
A technology for measuring information on a subject by use of a terahertz wave is generally as follows. That is, a pulsed light output from a light source (for example, a femtosecond laser light source) is branched into two to be a pump light and a probe light by a branching part. The pump light between those is received by a terahertz wave generating element (for example, a nonlinear optical crystal or a photoconductive antenna element), and thereby, a pulsed terahertz wave is generated from the terahertz wave generating element. This generated terahertz wave is transmitted through or reflected by a subject, to acquire information (for example, an absorption coefficient, a refractive index) on the subject, and thereafter, the terahertz wave is made incident on a terahertz wave detecting element (for example, an electro-optic crystal or a photoconductive antenna element) in substantially the same timing as that of the probe light.
In the terahertz wave detecting element which receives the terahertz wave and the probe light, a correlation between both lights is detected. For example, in a case in which an electro-optic crystal is used as the terahertz wave detecting element, the terahertz wave and the probe light are coupled by a wave synthesizer, to be made incident on the electro-optic crystal, and birefringence is induced in accordance with propagation of the terahertz wave in the electro-optic crystal, and a polarization state of the probe light is changed by the birefringence. A change in the polarization state of the probe light in the electro-optic crystal is detected. As a result, an electric field amplitude of the terahertz wave is detected, which enables to acquire information on the subject.
With respect to acquisition of information on a subject with a terahertz wave, as disclosed in Patent Document 1, in some cases, acquisition of information on a subject with a terahertz wave is carried out, not only by transmission or reflection of a terahertz wave through or by a subject part, but also by making a terahertz wave be totally reflected by a plane of a prism to generate an evanescent component, and to irradiate the subject on the plane with the evanescent component of the terahertz wave. According to the description in Patent Document 1, the technology by utilizing total reflection of a terahertz wave yields advantageous effects that a subject is not limited to a solid substance and the like.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2004-354246    Patent Document 2: Japanese Published Unexamined Patent Application No. 2006-184078